1. Field of the Invention
The present invention is directed to a system and method of producing an undistorted dark field strain map of a semiconductor material at high spatial resolution through dark field electron holography.
2. Description of Related Art
Electron microscopes have been used extensively to look at structure at the nanometer scale. Most of the information obtained from electron microscopes is amplitude information. Yet, the phase information of electron microscopy, which can be obtained from off-axis electron holography, provides unique information on electronic structure and structural changes in a wide variety of materials. Monocrystalline silicon (Si) is used extensively in semiconductor devices, and has a diamond cubic crystal structure. Preferred electron flow direction in Si devices is along the <220> crystal direction. It is discovered that strain along this direction can either enhance or retard the current flow. For the semiconductor industry, junction profiling and strain mapping in silicon (Si) at high spatial resolution provide information that is critical for further scaling of semiconductor devices. Bright-field holography can measure the phase change of electrons traversing the materials, which is directly related to the mean inner potential of silicon, indicating the junction position at the nanometer scale. Furthermore, in recent years stressors have been incorporated into devices to change the semiconductor lattice constant in the channel region and thereby enhance hole and electron mobility. Like the junction definition, the extra processing steps involved to add strain in a device have increased development and manufacturing costs. One way to minimize development cycle time is to monitor, at a nanometer scale, changes in channel deformation resulting from process changes. Strain mapping along the <220> direction in Si provides channel strain information important to the semiconductor industry. For pFET, compressive strain improves device mobility; whereas for nFET, tensile strain improves device performance.
Dual-lens operation allows electron holography to be performed from low to high magnification and provides the field of view (FOV) and fringe spacing necessary for two-dimensional (2D) junction profiling and strain measurements for devices with various sizes. Prior art patents in this area include U.S. Pat. Nos. 7,015,469 and 7,102,145.